Pseudoin situconstruction of high-performance thermoelectric composites with a dioxothiopyrone-based D-A polymer coating on SWCNTs

Article abstract of DOI:10.1039/d0ra10625a

Organic polymer/inorganic particle composites with thermoelectric (TE) properties have witnessed rapid progress in recent years. Nevertheless, both development of novel polymers and optimization of compositing methods remain highly desirable. In this stud

Full text of DOI:10.1039/d0ra10625a

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Pseudo in situ construction of high-performance
thermoelectric composites with
Cite this: RSC Adv., 2021, 11, 8664
a dioxothiopyrone-based D–A polymer coating on
SWCNTs†
a
a
Wen-Qiang Qu,ab Cai-Yan Gao,
Ping-Xia Zhang,c Xin-Heng Fan
*
a
*
and Lian-Ming Yang
Organic polymer/inorganic particle composites with thermoelectric (TE) properties have witnessed rapid
progress in recent years. Nevertheless, both development of novel polymers and optimization of
compositing methods remain highly desirable. In this study, we first demonstrated a simulated in situ
coagulation strategy for construction of high-performance thermoelectric materials by utilizing single-
walled carbon nanotubes (SWCNTs) and a new D–A polymer TPO-TTP12 that was synthesized via
incorporating dioxothiopyrone subunit into a polymeric chain. It was proven that the preparation
methods have
a significant influence on thermoelectric properties of the TPO-TTP12/SWCNT
composites. The in situ prepared composite films tend to achieve much better thermoelectric
performances than those prepared by simply mixing the corresponding polymer with SWCNTs. As
a result, the in situ compositing obtains the highest Seebeck coefficient of 66.10 ꢀ 0.05 mV Kꢁ1 at the
TPO-TTP12-to-SWCNT mass ratio of 1/2, and the best electrical conductivity of up to 500.5 ꢀ
53.3 S cmꢁ1 at the polymer/SWCNT mass ratio of 1/20, respectively; moreover, the power factor for the
Received 18th December 2020
Accepted 10th February 2021
in situ prepared composites reaches a maximum value of 141.94 ꢀ 1.47 mW mꢁ1
K
ꢁ2, far higher than that
of 104.68 ꢀ 0.86 mW mꢁ1 Kꢁ2 for the by-mixing produced composites. This indicates that the
dioxothiopyrone moiety is a promising building block for constructing thermoelectric polymers, and the
simulated in situ compositing strategy is a promising way to improve TE properties of composite materials.
DOI: 10.1039/d0ra10625a
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Seebeck coefficient, but low thermal conductivity. Given low
thermal conductivities of organic materials or organic–inor-
Introduction
ganic composites, a simplied expression of power factor
dened as PF ¼ S2s is most oen used for evaluating their TE
performances.4
Thermoelectric materials, which can realize direct energy
conversion between heat and electricity with no moving parts,
no noise and a long operating lifetime even under very low
temperature gradients relative to the environmental tempera-
ture, have been considered as promising candidates to meet
current challenges of increasingly global energy shortages and
environmental pollution.1,2 The thermoelectric performance is
determined by the gure of merit (ZT ¼ S2sT/k), where S, s, T
and k are the Seebeck coefficient, electrical conductivity, abso-
lute temperature and thermal conductivity, respectively.3
Therefore, the high-performance thermoelectric materials are
supposed to have a high electrical conductivity and high
The organic–inorganic composites, composed of nano-sized
inorganic semiconductors and organic conducting polymers,
can produce dramatically enhanced TE performances through
synergistic effects resulting from high electrical conductivities
of inorganic components as well as large Seebeck coefficients
and low thermal conductivities of organic constituents.5–12 In
this respect, conducting polymers/carbon nanotube (CNT)
composites have demonstrated tremendous progress.13–22
Currently, TE performance optimization of the polymer/CNT
composites are concentrated on two aspects including the
molecular design of polymers and the preparation technology.
The preparation methodology for organic/inorganic
composites plays a vital role in determining performances of
composites. Generally, two approaches are adopted to form the
polymer/CNT TE composites. One is the in situ polymerization
of monomers in a dispersion of SWCNTs, which is applicable to
such classical conductive polymers as PANI, PPy, PTh, and
aBeijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green
Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China.
E-mail: gaocaiyan@iccas.ac.cn; yanglm@iccas.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing, P. R. China
cKey Laboratory of Science and Technology on High-tech Polymer Materials, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10625a
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PEDOT gotten easily by oxidative polymerization under simple
Despite these advances, the structural types of conjugated
conditions.13,14 The other is the drop-casting of a mixed polymers for TE composites are still extremely limited and their
suspension of the pre-prepared polymers and SWCNTs, which TE performances remain far below the expected level. Appar-
is suitable for those polymers that are structurally complicated ently, it would be very meaningful to develop novel types of
and synthetically strict (such as water-free and oxygen-free conjugated polymers and efficient compositing method for TE
conditions).16–22 Although many other lm-forming technolo- composite materials. Accordingly, 4-(dicyanomethylene)-2,6-
gies, such as dip-coating,23 spray-coating24 and spin-coating,25 diphenyl-4H-thiopyran-1,1-dioxide (Fig. 1a) and its precursor
were used for fabricating TE samples, almost all the recently- dioxothiopyranone (Fig. 1b) aroused our interest as a sulfone-
developed complicated TE polymer/CNT composites have containing analogue of tetracyanoquinodimethane (TCNQ).28
been prepared through the direct drop-casting. However, the The dioxothiopyranone structure exhibits many advantages:29
drop-casting method is difficult to form uniform interface rst, the moiety is an approximately planar ring with only minor
coating layer of polymers on the surface of SWCNTs because of deviation, and a more rigid near-plane structure could be
the rapid evaporation of solvents and static growth of polymers formed through intramolecular interaction between the ortho-
on the substrate, leading to conglutination and lling of the hydrogen on the 2- and 6-phenyls and the oxygen atom of the
precipitated polymer in the gap of SWCNT networks. It is worth sulfone group, being helpful to the improvement of the carrier
mentioning that a promising coagulation method has long transport and the electrical conductivity; second, its electron-
been ignored in the preparation of thermoelectric materials, decient nature, reinforced synergistically by strongly
although it was widely used in nanotechnology and materials electron-withdrawing carbonyl or dicyanomethylidene group
science because it can provide a better dispersion of inorganic and the sulfone group, offers the possibility for constructing
particles in a polymer matrix26 and a uniform coating layer of a D–A type conjugated polymer, which may be benecial to the
polymers on the surface of inorganic particles.27
enhancement of carrier mobility of the polymer on account of
Another urgent and arduous task is to develop new types of intramolecular charge transfer; and last but not least, the
conjugated polymers for TE polymer/CNT composites. In the moiety is ready to undergo molecular modication or derivati-
past years, a number of new structural segments and special zation at its 2- and 6-positions, opening a pathway to enrich
bonding modes were used for construction of more structurally organic TE materials. In brief, dioxothiopyranone unit is a very
complicated polymers than the classical polymers, and those promising building-block for construction of high-performance
conducting polymers were employed to form the composites polymeric thermoelectric materials. However, there has yet
with superior TE performances.15–22 In 2016, our group reported been no report on the dioxothiopyranone-based polymers and
a kind of poly-Schiff base/SWCNT TE composites and the their application in TE materials so far, although its small-
chelation effect of the Schiff base with transition metal ions on molecular derivatives served as excellent electron-transporting
the TE performance.15 Subsequently, Wang et al. prepared materials in electrophotography in the mid of 1980s.30
a series of TE composite lms based on SWCNTs and
We designed and synthesized a novel thiopyranone-based
bipyridine-containing polyuorene derivatives, discussing the D–A alternating conjugated polymer (coded as TPO-TTP12) by
variation trend of TE performance caused by chelation of the Stille coupling polymerization. Thereaer, two series of
bipyridine unit with various metal ions.17 It is worth mentioning polymer/SWCNT composites were prepared and evaluated
that Wang et al. made a great deal of work for developing new taking advantage of, respectively, the simulated in situ coagu-
polymer/SWCNT composites with high TE performances. lation method (abbreviated as “in situ method”) and the
Notably, a benzodithiophene (BDT)-based conjugated polymer directly-mixing preparation method (abbreviated as “by-mixing
(PBDT-EDOT) was developed, and the thermoelectric behavior method”). The results show that introduction of dioxothiopyr-
of its composite lms with SWCNT was investigated elaborately, anone is effective to elevate the Seebeck coefficients of the
with a maximum power factor of 74.6 mW mꢁ1 Kꢁ2 for the composites. By comparison, the in situ method makes a well-
SWCNT content of 90% at 400 K.18 Making use of olen balance between the electrical conductivity and the Seebeck
metathesis, the cross-linking effect on thermoelectric proper- coefficient of the composites, achieving signicantly high See-
ties of polymer/CNT composite lms have been studied.19 beck coefficients with a maximum value of 66.1 ꢀ 0.05 mV Kꢁ1
.
Consecutively, a class of self-assembled alkyl chain-linked As a result, the in situ prepared composite lm exhibits the
naphthalenediimide (NDI)/SWCNT composites were systemati- highest power factor of 141.9 ꢀ 1.5 mW mꢁ1 Kꢁ2, which is nearly
cally examined, achieving the best power factor of 237.6 ꢀ 20.8 30% higher than that of the by-mixing prepared sample. To the
mW mꢁ1 Kꢁ2 20 Very recently, by controlling interfacial doping of
.
7,7,8,8-tetracyanoquinodimethane (TCNQ) deposited in
vacuum, a type of high-performance thermoelectric composites
of SWCNTs and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothio-
phene (C8BTBT), were developed.21 Almost simultaneously, Qiu
et al. reported a new class of thermoelectric composites by
combining SWCNTs and a BDT-based D–A polymer with
carbazole segment as the side-chain. Both power factors
reached about 60 mW mꢁ1 Kꢁ2 before and aer doping
composites with F4TCNQ at room temperature.22
Fig. 1 Typical structure of (a) dioxothiopyranone-based electron-
transporting materials and (b) dioxothiopyranone substituted at the 2-
and 6-positions.
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best of our knowledge, it is the rst time that the simulated in alkyl chains increases signicantly the solubility of polymers in
situ coagulation strategy was used for fabrication of polymer/ common organic solvents. In particular, the combination of
CNT TE composites, and it may be expected to become alternating dioxothiopyranone acceptor with terthiophene
a universally applicable means to develop high-performance donor would be expected to enhance the carrier mobility due
polymer/CNT thermoelectric composites.
likely to strong intramolecular charge transfer effects.
Fabrication of polymer/SWCNT composite lms
Results and discussion
Scheme 2 illustrates two different strategies for the fabrication
of polymer/SWCNT composite lms. A simulated in situ prep-
aration procedure (the in situ method) is depicted in Scheme 2A:
rstly, the solution (a) and the dispersion (b) were prepared
respectively by dissolving the polymer TPO-TTP in anhydrous
dichloromethane and dispersing homogeneously SWCNTs in
ethanol; then the solution (a) was added drop-wise into the
dispersion (b) with a syringe so that the polymer precipitated
out slowly and wrapped on the surface of SWCNTs; and aer
slow evaporation of CH2Cl2 with stirring for 24 h, the residual
suspension was ltrated by suction to give the composite lms
with a uniform surface. For comparison, a directly-mixing
procedure (the by-mixing method) was performed as
described in Scheme 2B. In contrast, such a by-mixing method
led to the poor TE performance of composite lms because of
too quick precipitation of the polymer from solution (a) on the
SWCNTs. However, all the composite lms prepared by two
methods show high thermal stability and good exibility (as
shown in Fig. S23 and S24 in the ESI†).
Preparation of the polymer and its composite lms with
SWCNTs
The synthetic route to the polymer and the preparation process
of the polymer/SWCNT composite lms are shown in Schemes 1
and 2, respectively (the detailed synthesis procedure and char-
acterization are described in the ESI Section†).
Design and synthesis of the polymer TPO-TTP12
As demonstrated in Scheme 1, the polymer TPO-TTP12 was
synthesized by the Stille coupling reaction between 1,1-dioxo-
thiopyranone substituted by 4-bromophenyl (compound 4)
and ditin reagent of terthiophene having two dodecyl side
chains (compound 7). Following the previously reported refer-
ence with minor modication,30 the intermediate 4 as acceptor
segment was achieved via a series of reactions from the
condensation of 4-bromobenzaldehyde and acetone, to the
addition of sodium hydrosulde hydrate to dienone, the
oxidation of tetrahydrothiopyran, and the dehydrogenation of
sulfone. The intermediate 7 as donor fragment was obtained by
the Stille coupling between the corresponding bromide and tin
reagent. Apparently, the symmetry of dodecyl-substituted ter-
Photophysical and electrochemical properties and theoretical
simulation
thiophene simplies the structural characterization of the To facilitate understanding, the structures of the polymer and
polymer to some extent. Meanwhile, the incorporation of long its D and A units are displayed in Fig. 2a. The UV-vis absorption
Scheme 1 The synthetic route to the polymer TPO-TTP12.
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Scheme 2 Two preparation processes of the TPO-TTP12/SWCNT composite films.
spectra of their CH2Cl2 solutions and solid lms are shown in stronger molecule aggregations resulting from the enhanced
Fig. 2b. In the dilute dichloromethane solution, both TPO as intermolecular p–p interactions in the solid lm. Simulta-
acceptor (A unit) and TTP12 as donor (D unit) exhibit similar neously, the corresponding optical bandgap was calculated to
lm
absorption bands in the range of 300–400 nm originating from be about 1.81 eV using the formula Eogpt ¼ 1240/l
according
onset
the p–p* absorption. Compared to the spectra of individual to the onset absorption edge of the polymeric lm at 685 nm.
fragments, the spectrum of TPO-TTP12, constructed from D and
The electronic energy levels of the polymer were investigated by
A units, shows a strong absorption in the broad region of from cyclic voltammetry using the ferrocene/ferrocenium (Fc/Fc+) as
300 to 700 nm, which can be ascribed to the strong intra- internal calibration, which has an oxidation potential of 4.8 eV at
molecular charge transfer (ICT) transition from the electron- vacuum level. As shown in Fig. 2c, the highest occupied molecular
donating unit (TTP12) to the electron-withdrawing part orbital (HOMO) level of the polymer is estimated to be ꢁ5.44 eV
(TPO).31 By contrast, a red-shied broadening of around 60 nm from the onset oxidation potential according to the equation
ox
+
+
can be observed for the solid-state samples on account of the EHOMO ¼ ꢁ(4.80 eV ꢁ e41/2,Fc/Fc + e4onset), where 41/2,Fc/Fc is the
Fig. 2 (a) The molecular structures of D and A units and the polymer TPO-TTP12; (b) UV-vis absorption spectra for the D and A units and the
polymer TPO-TTP12 in CH2Cl2 solutions (solid lines) and solid films (dot line); (c) cyclic voltammetry curves of the polymer film with ferrocene as
the internal standard substance; (d) the optimized geometry for the lowest energy conformation; (e and f) the DFT-calculated isosurfaces of the
HOMO (e) and LUMO (f) orbital densities of the polymer at the GGA/BLYP level.
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half wave potential of Fc/Fc+ versus Ag/AgCl in the measurement assigned to the stretching vibration of saturated C–H bonds.32
system (0.44 V). The lowest unoccupied molecular orbital (LUMO) The characteristic absorption arising from the C]O stretching
energy level of the polymer can be calculated to be ꢁ3.63 eV from vibration appears at a distinct lower wave number of 1643 cmꢁ1
the equation of ELUMO ¼ EHOMO + Eogpt. Evidently, a low LUMO due to its conjugation with two adjacent C]C bonds.33 A set of
energy level for polymers would be very benecial to p-doping on bands at 1600 cmꢁ1, 1571 cmꢁ1 and 1546 cmꢁ1 are dened as
SWCNTs.
the C]C stretching vibration of the aromatic ring.34 The sharp
Density functional theory (DFT) calculations were performed peaks at 1308 cmꢁ1 and 1129 cmꢁ1 are corresponding to the
to investigate the molecular conguration and electronic characteristic absorption caused by the stretching vibration of
structure of the polymer TPO-TTP12, which possesses two main S]O in sulfone.28 Additionally, the bands at 1197 cmꢁ1 and
conformations due to the rotation around the bonds that link 829 cmꢁ1 are identied as the in-plane and out-of-plane
the acceptor unit and donor unit. For ease of analysis, the dimer aromatic C–H bending vibrations for the 1,4-disubstituted
with two repeated units was chosen as a model system aromatic groups.35 In a word, the FT-IR spectrum shows the
(Fig. S25†). Different from common linear and zigzag geome- characteristic absorptions of the important functional groups
tries, the lowest energy geometry is a nearly circular structure for the polymer, preliminarily conrming its successful
(Fig. 2d), which may probably be attributed to the unique synthesis.
geometric angle in the TPO unit. In addition, the calculated
In order to ensure the chemical structure of the polymer
isosurfaces of the HOMO and LUMO orbital densities are shown remain intact aer compositing with SWCNTs, the IR spectra
in Fig. 2e and f, respectively. The HOMO orbital principally for pure SWCNTs and some representative composite lms are
concentrates on the donor segment with three thiophene rings also displayed in Fig. 3b with the reappearance of the IR spec-
at one end of the dimer, but extends over the neighboring trogram of the polymer. It is quite clear that all those compos-
linking benzene ring, whereas with smaller contributions. In ites exhibit spectral absorption patterns similar to the pristine
contrast, the LUMO orbital is mainly localized on two 1,1- polymer, without new peaks appearing. This suggests that the
dioxothiopyranone segments (TPO) with the electron- chemical structure of the polymer is relatively stable without the
withdrawing nature, involving slightly the adjacent phenyl occurrence of structure damage during the combination with
rings. From the distribution diagrams of wave functions, it can SWCNTs. However, it should be noted that the absorption
be deduced that the alkyl chains of the polymer have hardly any bands, respectively, at 1643 cmꢁ1 for C]O and at 1308 cmꢁ1
contribution to the frontier molecular orbits. As a result, the and 1129 cmꢁ1 for S]O, are obviously blue-shied to higher
HOMO and LUMO levels were calculated to be ꢁ4.796 eV and wavenumbers. This should be a compromising consequence of
ꢁ3.654 eV, respectively, and the corresponding band gap was two factors: on one hand, the intermolecular hydrogen bonding
acquired to be 1.142 eV from the equation DE ¼ EHOMO ꢁ ELUMO
.
in the polymer weakened with decreasing the polymer concen-
Besides, the calculated charge population shows that the oxygen tration by introduction of SWCNTs, leading to a strong blue-
atoms of sulfone and carbonyl in TPO carry negative charges, shi of the absorption peaks; simultaneously, a modest red-
implying that these atoms can attract electrons from the poly- shi occurs due to the reduced force constants for C]O and
mer backbone, and thus promote the electron transfer from S]O, which are caused by the electron transfer from the
SWCNTs to the polymer.
SWCNT system to the electron-decient polymeric backbone
arising from the interfacial interactions between the polymer
and SWCNTs. As a result, a relative blue shi of the absorption
peaks can be observed since their blue-shis are more prom-
inent than the red-shis. Furthermore, the degree of blue shi
increases with the increase of SWCNT contents. Especially,
compared to the by-mixing prepared composite lms, a more
remarkable blue-shi for the in situ prepared composites can be
IR and Raman spectra
Fourier transform infrared spectroscopy (FT-IR) is a simple and
effective means for analysis of polymeric structures. The FT-IR
spectrum of the polymer TPO-TTP12 is shown in Fig. 3a. The
strong absorption bands at 2923 cmꢁ1 and 2851 cmꢁ1 are
Fig. 3 FT-IR spectra of (a) the polymer TPO-TTP12 and (b) its representative composite films; (c) Raman spectra of the representative composite
films.
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observed since the stronger p–p interfacial interaction is viewing, the polymer/SWCNT composite lm with mass ratio of
induced by the more uniform wrapping of the polymer on the 20/1 is taken as a typical example. The images for the in situ
surface of SWCNTs. The Raman spectra of the pure SWCNTs prepared composite lm with different magnications are dis-
and several typical composite lms are shown in Fig. 3c to study played in Fig. 4b–d. From Fig. 4b, it can be seen that the
the interfacial interactions between the polymer and SWCNTs morphology of the composite is apparently inuenced by the
in composite lms. The pure SWCNTs present strong charac- introduction of SWCNTs, merging quasi-spheres of the polymer
teristic G bands at 1573 cmꢁ1 and 1593 cmꢁ1, respectively, and bers of the SWCNTs into a porous network structure with
which arise from the stretching vibration of sp2-hybridized the pearl-necklace-like morphology, which is formed by the
carbon atoms.36 In contrast, the polymer/SWCNT composite interfacial attractions from the p–p interactions and van der
lms reveal a gradual blue-shi of the G-band peaks with Waals forces between SWCNTs and the polymer. This
increasing polymeric contents, implying the effective charge morphology is strikingly different from the previously reported
transfer from the SWCNTs to the D–A type of polymeric mole- twining sphere-wire structure formed by the surface adhesion of
cules.37,38 In particular, the G peak at 1593 cmꢁ1 for the pure SWCNTs on the spheres of polymers.15 Moreover, the unique
SWCNTs shis to by 5 cmꢁ1 higher wave number, appearing at morphological feature is further accentuated as shown in the
1598 cmꢁ1 in the composite lm with polymer/SWCNT mass TEM image of Fig. 4e, which is in sharp contrast to the thinner
ratio of 20/1. Unfortunately, the ideal Raman spectrum of the and smoother TEM morphology for the pure SWCNTs
pure polymer failed to be recorded due to the overlap of the (Fig. S26†). With changing the enlargement factor from high to
strong uorescence emission signals and the weak Raman low magnications, a small cellular structure with relative
scattering signals at an excitation wavelength of 514 nm. In the uniform pores is presented in Fig. 4c. Interestingly, a fairly
acquired spectra, because of the strong uorescence interfer- smooth and homogenous morphology appears at smaller
ences of the polymer, the background noises of the composite magnication, as demonstrated in Fig. 4d. For comparison, the
lms were synchronously enhanced with the increase of poly- images with the corresponding magnications for the by-
meric contents, leading to the indiscernibility of some charac- mixing produced polymer/SWCNT composite lm with the
teristic peaks; on the other hand, there remained several mass ratio of 20/1 were shown in Fig. 4g–i. Comparing with
important vibration modes appearing in the spectra of some Fig. 4b and g gives a more disorder network morphology with
composite lms. An extremely weak band at 1190 cmꢁ1 is abundant bare SWCNTs, in which bulky grains of the polymer
assigned to the characteristic peak of S]O stretching vibration are sporadically scattered. As expected, a more uneven and
of the sulfone. The band at 1341 cmꢁ1 originates from the C–C irregular topography stands out at lower magnication, as
stretching vibration of the p–p conjugated backbone.19 The shown in Fig. 4h and i. Especially, the morphology with large
bands of the C]C symmetrical stretching vibration appear at irregular bulges in Fig. 4i is in stark contrast to that of Fig. 4d. It
1413 cmꢁ1 and 1449 cmꢁ1, respectively.19 Also, the asymmetric is surprising that the morphology presented in Fig. 4e is obvi-
C]C stretching mode is found at 1507 cmꢁ1 18 In addition, the ously different from that in Fig. 4j in which some bulky grains
.
weak peak at 1634 cmꢁ1 is denitely attributed to the stretching are adhered to bare SWCNTs with almost no coating of the
vibration of the C]O conjugated with two adjacent C]C polymer on the surface of SWCNTs. Thus, it can be well
bonds.39 At the same time, it should be noted that the in situ understood that the preparation method for composite lms
prepared composites show slightly obvious vibration signals in has a huge impact on their microscopic morphologies.
comparison to those of the by-mixing prepared ones. This is
because their uorescence interferences is attenuated more
effectively, which is caused by the diluted concentration of the
XPS spectra
polymer resulting from the more effective interactions between X-ray photoelectron spectroscopy (XPS) was adopted to investi-
polymeric molecules and SWCNTs.
gate the compositions of materials and changes in the binding
energies for the key elements. The polymer/SWCNT composites
with the mass ratio of 20/1 were taken as representative exam-
ples with the pure SWCNTs and pristine polymer as controls.
Surface morphologies
As well known, the properties of materials depend on the Notably, in comparison with the XPS spectrum of the SWCNT
structures, including their molecule-level structures and lm, both the pristine polymer and its composites show a new
micromorphology. All the morphologies for the polymer, energy peak at about 164 eV coming from the S 2p of the poly-
SWCNTs and the polymer/SWCNT composites were examined mer, conrming the successful compositing of the polymer and
to understand the relationship between the micro-structure and SWCNTs (shown in Fig. 5a). From the deconvolution spectrum
performance of the composites through both the eld-emission of S 2p of the pristine polymer provided by Fig. 5b, two set of S
scanning electron microscope (FESEM) and transmission elec- 2p doublets, originating from two species of S in the polymer,
tron microscope (TEM).
are clearly visible. One S 2p doublet at low binding energy is
Several selected morphology images are shown in Fig. 4. separated into S 2p3/2 at 163.65 eV and S 2p1/2 at 164.83 eV,
Fig. 4a–f present respectively an adhesive sub-globose which are derived from thienyls in the polymer; the other S 2p
morphology formed via the agglomeration of scale-like gran- doublet located at higher binding energy is as well divided into
ular particles for the pristine polymer and a randomly inter- two peaks at 167.87 eV and 169.05 eV respectively, corre-
twined brillar morphology for the pure SWCNTs. For ease of sponding to the S 2p3/2 and S 2p1/2 of the higher oxidation state
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Fig. 4 FESEM images of (a) the pristine polymer; (b–d) the in situ produced polymer/SWCNT composite film with a polymer/SWCNT mass ratio
of 20/1 at different magnifications; (f) the pure SWCNTs and (g–i) the by-mixing produced polymer/SWCNT composite film with a polymer/
SWCNT mass ratio of 20/1 at different magnifications; TEM images of (e) the in situ produced and (j) the by-mixing prepared polymer/SWCNT
composite films with a polymer/SWCNT mass ratio of 20/1.
of sulfur in the dioxothiopyrone unit.40,41 In the composite lms, the more effective interaction between SWCNTs and polymer.
all of their XPS spectra display two set of similar S 2p doublets Considering only two characteristic species of oxygen atoms in
with only slight differences of both energy values and peak areas the polymer, the binding energies of O 1s for the polymer were
(as shown in Fig. S29 in the ESI†). In addition, a signicant also used for clarifying the charge transfer between the polymer
doping-induced change is discerned in both the C 1s and O 1s and SWCNTs. From the deconvolution spectrum of O 1s for the
core levels. The deconvolution C 1s spectra of sp2-hybridized pristine polymer in Fig. 5d, we can see that the original O 1s
carbon for the pure SWCNTs and the composite lms with the single peak is separated into two single peaks, including O 1s
polymer/SWCNT mass ratio of 20/1 are displayed in Fig. 5c. It is peak of carbonyl at 531.64 eV and O 1s peak of sulfone at
observed that the peaks of the C 1s for the by-mixing and the in 532.42 eV. Compared with the pristine polymer, the O 1s peaks
situ prepared composite lms are downshied, respectively, to of the by-mixing and the in situ prepared composite lms are
lower binding energy levels of 284.32 eV and 284.25 eV relative shied to the lower bonding energies from 532.42 eV to 532.33
to that of 284.41 eV for the pure SWCNTs, corresponding to and 531.91 eV for sulfone (Fig. 5e) and from 531.64 eV to 531.52
a change in the Fermi level of SWCNTs. This indicates that the and 531.09 eV for carbonyl (Fig. 5f), respectively, which may be
effective electron transfer occurs from SWCNTs to the polymer, explained for the enhanced electronic cloud density on the
namely that the polymer acts as an acceptor to attract electrons electronegative O atoms of carbonyl and sulfone resulting from
from the SWCNT backbone.42–44 And, the more distinct change the electron transfer from the SWCNT system to the polymer
in the binding energy for the in situ prepared composite reveals backbone.42–45 These results are exactly in concert with those of
Fig. 5 XPS spectra of (a) full-range for the pristine polymer, the pure SWCNT and the composite films prepared by two methods with a polymer/
2
SWCNT mass ratio of 20/1; (b–f) the deconvolution of (b) S 2p for the pure polymer, (c) C 1s of Csp for the pure SWCNTs and the composite films
prepared by two methods with a polymer/SWCNT mass ratio of 20/1, (d) O 1s for the pure polymer, (e) O 1s of sulfone and (f) O 1s of carbonyl for
the pure SWCNTs and the composite films prepared by two methods with a polymer/SWCNT mass ratio of 20/1.
8670 | RSC Adv., 2021, 11, 8664–8673
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Raman spectra and simulated calculations, revealing the strong a substantial contribution to the improvement of Seebeck
interfacial interactions between the polymer and SWCNTs.
coefficients of the composites. Additionally, the power factor of
the composites also shows a monotonous increase with the
increase of SWCNT content. It is mainly ascribed to a dramatic
increase in the electrical conductivity and a relatively moderate
uctuation in Seebeck coefficient with the decrease of the
polymer loadings. Unexpectedly, the best power factor of 141.94
ꢀ 1.47 mW mꢁ1 Kꢁ2 was obtained, which is far higher than that
of the pure SWCNTs (86.04 ꢀ 3.47 mW mꢁ1 Kꢁ2) and approxi-
mately two and a half times as large as the maximum value of
the poly-dodecylthiophene/SWCNT composites reported very
recently by Chen et al. (60 mW mꢁ1 Kꢁ2).47
Thermoelectric performance
To evaluate the TE performance of the composites, the key
parameters for organic thermoelectric materials, i.e., the elec-
trical conductivity (s), Seebeck coefficient (S) and power factor
(PF), were investigated as a function of the polymer/SWCNT
mass ratio. Note that the electrical conductivity and Seebeck
coefficient of the pure polymer cannot be measured directly by
the current four-probe method owing to its relatively poor
conducting performance.
As a comparison, Fig. 6b describes the variation tendency of
TE properties for the by-mixing prepared polymer/SWCNT
composites as the polymer/SWCNT mass ratio changes. Simi-
larly, the curve of electrical conductivities for these by-mixing
composites shows a monotonously upward trend, and main-
tains a steady linear growth within the entire range of mass
ratios. As a result of the dispersion of almost totally bare
SWCNTs in the polymer, Seebeck coefficients of the by-mixing
prepared composites should be more approximate to that of
SWCNTs because a large number of bare SWCNTs reinforce
their contribution. As expected, the curve of the Seebeck coef-
cient goes up rst and then down, with only a small uctua-
tion of around 10 mV Kꢁ1 from 37.64 ꢀ 0.54 mV Kꢁ1 to 50.02 ꢀ
0.04 mV Kꢁ1. Their power factors show a trend of rapid increase
with the increase of SWCNT contents in the range of low
SWCNT loadings, but keep almost unchanged aer a mass ratio
of 1/5. Finally, the highest power factor for such composites was
calculated to be 104.68 ꢀ 0.86 mW mꢁ1 Kꢁ2, which is 18 mV mꢁ1
Kꢁ2 or so higher than that of the pure SWCNTs (86.04 ꢀ 3.47 mW
mꢁ1 Kꢁ2).
To more intuitively compare the inuence of preparation
methods on TE performances, Fig. 6c presents the TE perfor-
mance of the composite lms prepared by two different strate-
gies at the same time. All Seebeck coefficients for the in situ
prepared composite lms are higher than those of the by-
mixing prepared ones in the case of the corresponding
polymer/SWCNT mass ratio, with a biggest difference of
approximate 20 mV Kꢁ1. The most essential cause for this big
gap between the Seebeck coefficients can be attributed to the
difference of their electron energy ltering effects. For the in situ
prepared composites, the uniform and dense coating of the
Firstly, effects of polymer/SWCNT mass ratios on TE prop-
erties in the in situ prepared composite lms are outlined in
Fig. 6a. The electrical conductivity of the composite lms
increases monotonously with the increase of SWCNT doping
contents, reaching a maximum value of 500.5 ꢀ 53.3 S cmꢁ1
aer experiencing a rst mild then sudden rise, which
approaches the conductivity of 555.85 ꢀ 10.1 S cmꢁ1 for the
pure SWCNTs. The reason should easily be understood. At
a high ratio of polymers to SWCNTs, the low conductive poly-
mer plays a leading role in limiting electrical conductivities of
the composites due to its dense and thick wrapping on the
SWCNTs' surface.46 But rather, the electrical conductivity of the
composites is primarily dominated by highly conductive
SWCNTs when the wrapping layer of the polymer on SWCNTs'
surface turns thinner with declining polymer/SWCNT mass
ratio, likely producing a rapid even steep rise. With regard to
Seebeck coefficients, they go through a process of rising rst
and then descending with the increase of the SWCNT content,
reaching a peak value of 66.10 ꢀ 0.05 mV Kꢁ1 at the polymer/
SWCNT mass ratio of 1/2, which is about 70% higher than
that of the pure SWCNTs (39.36 ꢀ 0.54 mV Kꢁ1). In principle, the
composite with the smallest loading of SWCNTs should give the
maximum Seebeck coefficient closest to that of the pristine
polymer. Nevertheless, due to the strong correlations between
the Seebeck coefficient and carrier mobility, low SWCNT load-
ings leads to low carrier mobilities and thereby low Seebeck
coefficients. As anticipated, the Seebeck coefficients begins to
smoothly fall aer the peak value with the decrease of polymer
contents, indicating a gradually weakening effect of the polymer
on the Seebeck coefficient of the composites. The above results
suggest that the polymer with dioxothiopyrone unit has
Fig. 6 Effects of (a and b) polymer/SWCNT mass ratios on the TE performance for (a) the in situ prepared and (b) the by-mixing prepared
polymer/SWCNT composite films; and (c) two compositing strategies on TE performances.
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nanostructured polymer on the surface of SWCNTs can effec- extended to other polymeric systems. In brief, this preliminary
tively enhance the electron energy ltering effect between research not only enriches the family of TE composites but also
interfaces, allowing the high-energy carriers to pass but block- opens
a new avenue for improving TE performance of
ing the low-energy carriers simultaneously. The resulting mean composites based on complicated polymers.
carrier energy in electron transport increases, leading to the
enhancement of the Seebeck coefficient.48,49 On the contrary,
due to the aggregation of a great deal of bare SWCNTs resulting
Conflicts of interest
from inhomogeneous and discrete wrapping of the polymer on There are no conicts to declare.
the surface of SWCNTs, the electron energy ltering effect for
the by-mixing prepared composites becomes weak signicantly
or even disappears. Differing from the Seebeck coefficients, two
Acknowledgements
curves of electrical conductivities from those composites The authors thank National Natural Science Foundation of
prepared by two strategies intersect each other to form an China (Project No. 21572235, 21503234, and 21402209) for
atypical olive-like shape. This result may be explained as nancial support of this work.
follows. The dense and thick enwrapping of polymer on the
surface of SWCNTs for the in situ prepared composites and the
crowded dispersion of polymer in the SWCNT network for the
Notes and references
by-mixing prepared composites result in their respective
approximate low electrical conductivity at the low loading of
SWCNTs. With the increase of SWCNT contents, the contribu-
tion of the exposed SWCNTs in by-mixing prepared composites
to the electrical conductivity gain gradually the upper hand over
that of the SWCNTs wrapped heavily with polymer in the in situ
prepared composites, causing two curves to diverge from each
other. As the content of SWCNTs continues to increase, the
thinning coating layer of polymer on the surface of SWCNTs for
the in situ prepared composites leads to a dramatic increase of
electrical conductivities, rendering two curves to get closer and
closer and nally converge. Consequently, the curves of the
power factors for two types of composites, regardless of prepa-
ration methods, show a variation trend similar to those of the
electrical conductivity, suffering a reversal at the 1/10 mass ratio
of polymer/SWCNT. Finally, the in situ prepared composite
achieves the largest power factor of 141.94 ꢀ 1.47 mW mꢁ1 Kꢁ2
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